Semiconductor chips may be analyzed for certain defects by detecting small amounts of light emitted by such defects. Typically, inspection is conducted either by low magnification lenses having relatively large numerical aperture to enhance light gathering capacity, or by higher powered microscopes operating at close spacing to the surface being analyzed. A chip may be scanned or imaged, and the result analyzed to determine defect characteristics and locations to enable design improvements and process quality control. While effective in some circumstances, these inspection methods and apparatus have significant limitations.
Both of the above detection techniques suffer from inherent limitations in light gathering capacity. Even a costly large aperture lens captures only a limited "cone" of light rays from an emission point, typically a small minority of light flux emitted. Further, in many circumstances, the light source is neither equally bright at all viewing angles nor emitted in a lambertian pattern. Often, light is emitted more laterally than axially. For instance, when there is significant metallization immediately above a light-emitting defect, there may be little or no detectable light emitted vertically (from the horizontal surface) or within the limited offset angle collected by conventional lenses; most or all emissions may be predonimately lateral.
High powered microscope objectives may have a moderately high acceptance angle, which is the angle subtended by the lens from the point being imaged. For emissions having a significant vertical distribution, these objectives may have acceptable light gathering capacity, because the optical axis is aligned vertically with the sample. Often, only a minority of rays are captured, limiting detection of sources at a low brightness threshold, or requiring larger dwell times during scanning to collect adequate light flux to activate sensors. Even where such optics perform adequately, they are unacceptable for inspecting large portions of packaged semiconductor chips near bond wires. Such bond wires protrude upwardly at the periphery of most chips, and are necessary to provide electrical connection between the chip and the external circuitry which stimulates light emissions from defects. High powered microscope lenses must be positioned closer to the surface being inspected than the typical height of bond wires. Thus, the significant diameter of such lenses prohibits their use for inspection of chip regions closer to wire bonds than one radius of the lens housing. Because such lens housing diameters may be comparable in size to chip dimensions, only a small central portion of many chips may be inspected by such microscopes.
The present invention overcomes the limitations of the prior art by providing an optical element for microscopic inspection of a surface. The optical element has an elongated body defining an optical axis, with a first end adjacent the surface and a second end directed toward an imaging instrument. The body has a curved reflective surface and an optical aperture at the first end, and defines first and second associated focal points on the optical axis. The first focal point is spaced apart from the first end of the body, such that positioning the surface at the first focal point generates an image of the surface at the second focal point. The reflective surface may be paraboloidal, with a concentrating lens focusing collimated rays to the second focal point, so that a conventional microscope may view the image generated at the second point.